Method of sealing nuclear reactor fuel elements having a casing made of high-chromium steel

ABSTRACT

The invention relates to nuclear power and can be used in manufacturing of fuel elements for nuclear reactors. A method of sealing nuclear reactor fuel elements is proposed comprising welding one end of a casing with a plug, both of high-chromium steel, loading the fuel element with fuel, and welding a second plug to another end of the casing. The casing is of a high-chromium ferrite-martensite steel and the plug is of a high-chromium ferrite steel. Argon arc welding is carried out at a volume ratio of the materials of the casing and the plug contributing to formation of the metal of the weld seam which allows formation of a ferrite phase in said metal, wherein the ratio is: V 1 /V 2 &gt;18, where V 1  is the volume of ferrite material and V 2  is the volume of ferrite-martensite material. Argon arc welding is carried out at a current of 14-20 A, a speed of 12-15 m/h, an arc voltage of 9-10 W and an argon flow rate of 7-8 1/min. This method provides for the desired quality of the welded joins and simplifies the fuel element manufacturing process.

The invention relates to nuclear power and can be used in themanufacture of fuel elements for power reactors.

There is a known method for sealing fuel elements, wherein the weldingis carried out by melting the casing end together with the plug usingthe non-consumable electrode in the atmosphere of shielding gases (U.S.Pat. No. 3,045,108).

The disadvantage of the method is in that the arc is ignited by thecontact between the electrode and the cylindrical projection of theplug, wherein tungsten inclusions get into the weld pool which canreduce the strength and, correspondingly, the quality of a weldingjoint.

The other known method of sealing fuel elements by fusion weldingcomprises welding the plug made of the same material as the fuel elementcasing to one end of the casing made of 1X18H10T (1Cr18Ni10Ti) stainlesssteel; fuel loading followed by welding the second plug to another endof the casing, thereby tightly sealing the fuel element containing thefuel (see the book: authors A. G. Samoylov, V. S. Volkov, M. I. Solonin“Fuel elements of nuclear reactors”, Moscow, Atomenergoizdat, 1996,analog). This method discloses all the essential technologicaloperations which are required to seal fuel elements, wherein anoscillator is used to ignite an arc.

In recent years, in view of the higher standards for performancecharacteristics of fuel elements of fast-neutron reactors (such as theradiation dose, coolant corrosive-erosive effect and temperature),high-chromium ferrite-martensite steels of the following grades: EI-852,EP-823, EP-900 are used in manufacturing of fuel elements casings.

The closest analog is the method of sealing fuel elements with thecasing made of high-chromium steel, which comprises welding the plugmade of the same material as the fuel element casing to one end of thecasing, fuel loading followed by welding the second plug to the otherend of the casing to form the weld joint assembly, and finally temperingthe weld joints to remove quenching structures therefrom (see the book“Development, production and operation of fuel elements of powerreactors” edited by Ph. G. Reshetnikov, vol. 2, Energatomizdat, 1995r,prototype).

The main disadvantage of such method of welding the steels of this gradeis a tendency to form quenching structures and cold cracking after acertain period of time upon completion of the welding in these steels.

A drawback of the method is that an additional operation is required forproducing high-quality weld joints for such steels, in particular, theweld joint tempering which is carried out at 740-760° C. for 20-30 min,keeping the time between welding operations and the subsequent temperingas short as possible. This operation carried out after tempering theweld seam is costly and increases complexity of the technology of fuelelement manufacturing, especially when tempering the weld joint whichseals the fuel element loaded with fuel.

The technical effect of the present invention is an increase in thedurability of the sealing of nuclear reactor fuel elements with casingsof high-chromium steels by means of providing a high-quality weldedjoint between a casing and a plug without subsequent heat treatment ofthe weld seam, which simplifies the manufacturing process.

This technical effect is provided by the method of sealing nuclearreactor fuel elements, comprising welding one end of the casing to theplug, both of high-chromium steel, loading the fuel element with fuel,and welding a second plug to another end of the casing, wherein thecasing is of a high-chromium ferrite-martensite steel and the plug is ofa high- chromium ferrite steel, wherein the argon arc welding is carriedout at a volume ratio of the materials of the casing and the plugcontributing to the formation of the metal of the weld seam which allowsformation of a ferrite phase in said metal, wherein the ratio is:

V ₁ /V ₂≥0,18,

-   -   where V₁ is the volume of ferrite material;    -   V₂ is the volume of ferrite-martensite material.

Argon arc welding is carried out at a current of 14-20 A, a speed of12-15 m/h, an arc voltage of 9-10 W and an argon flow rate of 7-8 l/min

The volume ratio of ferrite and ferrite-martensite materials in the weldjoint metal, which is ≥0.18, is based on the fact that a stable ferritephase is formed at values higher or equal to the designated value in thejoint melt zone, and the ferrite phase not tending to the cold crackingand not requiring tempering of the weld joint; at lower values thedesired ferrite phase is not formed, making necessary the heat treatmentof the weld joint metal.

The Figure illustrates the structure of the weld joint assembly whichincludes a casing 1 comprised of ferrite-martensite steel and a plug 2comprised of ferrite steel.

EXAMPLE

The method of sealing nuclear reactor fuel elements comprises weldingone end of a casing to a plug, both comprised of high-chromium steel,loading the fuel element with fuel, and welding a second plug to anotherend of the casing.

Modeling the welding process at different process variables, such as ashoulder width a, a casing thickness δ, and a predetermined penetrationdepth b, was carried out on a geometrically flat model of the weld jointassembly before and after welding, followed by determining the phasesformed in the weld joint by means of structural analysis, which allowscalculating ratio of the areas of steels of various grades participatingin forming the joint metal, to obtain the ferrite phase therein.

The resulting surface areas of materials of various grade weredetermined in slices of weld joints taking into account their fractionsin the formed joint metal and was conducted by means of the computerprogram JpSquare (LProSoft), and the phase formed in the weld jointmetal was determined by structural analysis. The volume of the phaseformed in the joint metal was proportional to the area of the metal inthe geometrical flat model of the weld joint assembly. The ratio ofmaterial volumes for the casing and the plug required to form theferrite phase in the joint metal is determined on the basis of thecalculated area of the materials to be coupled.

V ₁ /V ₂≥0.18,

where: V₁ is the volume of ferrite material;

-   -   V₂ is the volume of ferrite-martensite material.

The method of sealing is carried out by argon-arc welding with the useof a casing made of ferrite-martensite steel of the EP-823 grade with adiameter of 9.3 mm having a wall thickness of 0.5 mm

Ferrite steel of 05X18C2BΦAIO (05Cr18Si2WVNAI) grade was used for theplug, the shoulder width was 0.8 mm, its diameter corresponded to thecasing diameter (see Figure).

The chemical composition of steels used for welding is shown in Table 1.

TABLE 1 Chemical composition of the welded materials. Elements content,wt % Steel Cr Si W Nb Wo V Mn Al Ni N C Ce EP-823 10-12 1.1-1.3 0.3 0.30.6-0.9 0.2-0.4 0.5-0.8 — 0.5-0.8 ≤0.05 0.14-0.18 — 05X18C2MBΦA 

18.0 1.7 0.6 0.2 0.7 0.3 0.1 0.2 — 0.04 — 0.01 (05Cr18Si2MoWVN Al)

Mechanical characteristics of the steels are shown in Table 2.

TABLE 2 Mechanical characteristics of the welded material Mechanicalcharacteristics Mechanical characteristics at at 2° C. 650° C. Steelσ_(V), MPa σ_(0.2), MPa δ, % σ_(V), MPa σ_(0.2), MPa δ, % EP-823 829 67220 294 265 48 05X18C2MBΦA 

600 482 9 200 186 40 (05Cr18Si2MoWVNAl)

The conducted experiments with the steel 05×18C2MBΦAIO(05Cr18Si2MoWVNAI) demonstrated its high mechanical performance andperspective corrosion resistance in lead coolant (RU Patent 2238345“Steel for core fuel elements of lead-coolant nuclearreactors”/Velyuhanov V. P., Zelenskiy G. K., Ioltuhovskiy A. G.,Leontieva-Smirnova M. V., Mitin V. S., Sokolov N. B., Rusanov A. E.,Troyanov V. M.; the applicant and the patent holder—Russian Federationrepresented by Federal Agency on Atomic Energy, SSC VNIINM.; publ. Dec.20, 2004.)

The mode of welding the casing made of steel EP-823 with the plug madeof steel 05X18C2MBΦAIO (05Cr18Si2MoWVNAI):

-   -   welding current 15A,    -   welding velocity 14 m/h,    -   arc voltage 9 V,    -   argon rate 8 l/min

Following the technology described above, fuel element simulators weremade.

Metallographic researches of the weld joints for selected welding modeshave shown that the volume ratios of the ferrite phase to theferrite-martensite phase is from 0.46 to 0.51.

Mechanical strength tests have shown that the rupture of the samplesoccurs across the casing of fuel element simulators. Strength limit ofthe casing is 818 MPa.

Weld joints have been tested for corrosion in lead coolant for 4000hours. It has been found that the corrosion resistance of weld jointsremains at the level of corrosion resistance of the fuel casing.

The tests of the weld joints for gas-tightness carried out using ahelium leak detector by a mass spectroscopic method at a roomtemperature have shown that all joints are hermetically sealed.

The use of the inventive method of sealing fuel elements having a casingmade of high-chromium steel allows to improve the quality of the weldjoint between the casing and the plug and to greatly simplify sealingtechnology.

What is claimed is:
 1. A method of sealing nuclear reactor fuelelements, comprising welding one end of a casing to a plug, bothcomprised of high-chromium steel, loading the fuel element with fuel,and welding a second plug to another end of the casing, characterized inthat the casing is comprised of a high-chromium ferrite-martensite steeland the plug is comprised of a high-chromium ferrite steel, argon arcwelding is carried out at a volume ratio of the materials of the casingand the plug contributing to the formation of the metal of the weld seamwhich allows formation of a ferrite phase in said metal, wherein theratio is:V ₁ /V ₂≥0.18, where: V_(i) is the volume of ferrite material; V₂ is thevolume of ferrite-martensite material.
 2. The method of claim 1,characterized in that the argon arc welding is carried out at a currentof 14-20 A, a speed of 12-15 m/h, an arc voltage of 9-10 W and an argonflow rate of 7-8 l/min.